The inotropic effect of most agents is determined by the extent to which they influence Ca2+ release from the sarcoplasmic reticulum (SR). The use of the fluorescent indicator indo-1 provides a previously unobtainable degree of resolution of intracellular ionic measurements and an excellent means to examine drug actions on Ca2=1 metabolism. The proposed studies will examine the effects of several classes of inotropic agents, all of which also change Na+1. on Ca2+ release from the SR and force. The effects of these clamp measurements of membrane current and optical measurements of cell contraction in isolated dog ventricular myocytes. The overall goals of these experiments are to define 1) the direct actions of these agents on SR Ca2+ release; 2) indirect drug actions on force via alteration in Na+i and, therefore Ca2+1 via Na-Ca exchange; 3) the extent to which alterations in SR function contribute to drug inotropy and/or toxicity, and 4) the Na+i-Ca2+i-tension relation in ventricular muscle. The following specific questions will be addressed: 1) What is the basis for the disparity between toxic: therapeutic ratios for different cardiac steroids? 2) Do some or all cardiac steroids possess a direct action on the SR to promote inotropy and/or toxicity? 3) Do clinically useful local anesthetic antiarrhythmic agents produce negative inotropy by direct interference with SR release of Ca2+? 4) Does the Na+ channel toxin Anthopleurin-A possess a novel positive inotropic action that involves alterations in SR Ca2+ release? 5) Does dantrolene act like ryanodine to inhibit Ca2+ release from SR by a direct mechanism? 6) What are the quantitative relationships between Na+i, Ca2+i- and cardias force in ventricular muscle? The significance of these experiments is three-fold: first distinguishing between (direct) Ca2+i- and (indirect Na+1- dependent inotropic drug influences will improve our understanding of therapeutic and toxic actions of these clinically important drugs: second, defining the normal processes involved in determinants of contraction. This will provide some basis for defining abnormal (e.g. cardiomyopathy and heart failure). Third, defining drug actions on subcellular mechanisms will help us to devise more specific approaches to the development of new positive inotropic agents for the treatment of these diseases.